Place the mushrooms in a small bowl and pour the boiling water over them. Cover with a small saucer to submerge them completely and set aside for 30 minutes. Stem the mushrooms and thinly slice the caps, reserving soaking liquid.

Put the kombu and 7 cups of water in a large soup pot. Pour the mushroom soaking liquid into the pot, stopping before you get to any sediment in the bottom of the bowl. Heat over medium-high heat; as soon as the water comes to a simmer, remove the kombu and discard it. Reduce the heat to low.

Vegetarian Option - Ladle 2 cups of the stock into a small saucepan. Whisk 1 tablespoon of the miso into the stock. Stir in one quarter of the mushrooms and the green onions. Carefully add the tofu cubes; keep warm over medium low heat.

Add the bonito flakes to the remaining stock. As soon as they sink to the bottom, strain the stock through a fine-mesh sieve. Discard the bonito flakes, and return the stock to the pot. Add the remaining mushrooms and all the clams, bring to a simmer, and cook until the clams open, about 3 minutes. Discard any clams that do not open. Whisk the remaining 3 tablespoons of miso and green onions into the soup.

Heat the oil in a 3-quart saucepan over medium-high heat until shimmering hot. Add the curry paste and cook, stirring frequently, until fragrant, 20 seconds. Take the pan off of the heat and stir in the snap peas, shallots, pepper, coconut milk, lime leaves (if using), sugar, and 1 cup water. Stir to combine.

Add the tofu. Bring to a simmer over medium heat, cover, and cook until the vegetables are tender, 8 to 10 minutes. Stir in the basil, soy sauce, and lime juice. Let rest off the heat for 5 minutes. Season to taste with salt.